A lie on narrowest interpretation means a deliberate, knowing falsehood intended to deceive. However, in a wider sense, it is also accepted to mean something untrue through error, and something misleading or giving a false impression, with perhaps less strictness on absolute truth / false binary.
It's a lie on the most technical basis simply by the fact he said suicides "definitely" would be higher than covid-19 casualties with more lockdown. This is untrue - there's no guarantee at all. But it's a lie also in a looser, more important sense of deception.
Trump is perhaps more accurately a bullshitter - someone who just says what he feels like with no apparent interest or knowledge in whether it's not true or not. He can as easily be called a liar by the valid looser definition, even if he doesn't know that what he's saying is untrue. The second element is that he's making a prediction, which is not a fact to be obviously true or false at the point spoken to be clearly definable as a lie: on the narrowest definition. However, again, at a looser level, is this giving a misleading or false impression? It sure is. Here there is the relevance of the stats: in the USA, annual suicides are so hugely dwarfed by covid deaths that the uptick in suicides would have to be truly staggering. Given the sheer unlikelihood of this, it is more deceptive than realistic. A lie, then.